1. Field of the Invention
This invention relates to cutting tools. More particularly, this invention relates to a coated carbide tap for threading steels and other ferrous based alloys.
2. Description of the Related Art
It is well known that cutting tools generate temperatures that are high enough to limit the life of the tool, thereby reducing the useful cutting speed. There exists a variety of materials used to manufacture cutting tools. Cutting tools are often manufactured from a class of tool steels known as high-speed steels. Although high-speed steels have excellent strength and toughness, they have only moderate resistance to temperature compared to materials that are more refractory.
Cemented tungsten carbide is favored as a material for manufacturing cutting tools over tool steels such as high-speed steel owing to properties such as higher hardness and high temperature stability including the ability to retain hardness at high cutting temperatures. Typically cutting tools manufactured from cemented carbide can be used at cutting speeds that are at least three times higher than tools manufactured from “high-speed” steel and the life of the tool is longer. However, cemented tungsten carbide has lower fracture toughness and strength than high-speed steel and this limits its use in some machining operations.
One of the machining operations that is used to generate an internal screw thread is known as tapping. There currently exist two tapping methods. The dominant tapping method is by cutting and removing material from the walls of a hole to produce a V-shaped helical screw thread. Alternatively, internal screw threads can be created by displacing material to form an internal screw thread. However, forming taps require much higher power than cutting taps and produce an imperfect thread crest at the thread's minor diameter. For this reason, cutting taps are often preferred over forming taps.
Unlike most other cutting tools used, for example, in turning, milling, drilling and reaming, tapping tools have relatively weak cross sections and additionally the cutting edges are weak. For this reason, taps manufactured from cemented carbide according to current technology have only very limited use. When carbide taps of current designs are used, cutting edges chip or fracture easily, rendering the tool useless, even in relatively easy to machine materials, such as steel. For this reason, carbide taps of the current technology are limited to tapping materials that are even easier to cut than steel, such as aluminum and cast iron. Steel and other ferrous based alloys are the most frequently used material for assemblies requiring screw threads. Therefore, a tap manufactured from cemented tungsten carbide would find advantages in comparison to taps manufactured from high-speed steel if the cutting edge chipping and breakage problems could be overcome.
Cemented carbide grades consisting of tungsten carbide alloyed with other carbides, such as tantalum carbide, titanium carbide and niobium carbide in amounts over approximately 3% by weight have high wear resistance when used as a tool material for cutting steel and other ferrous based alloys. However, such carbide grades do not have sufficient strength and toughness to be used as a material for taps. Alternatively, grades containing principally tungsten carbide cemented with cobalt in concentrations over 10% by weight have high strength, but their wear resistance is low in steel machining, and therefore they are not suitable as a tool material for tapping steel.